A Cable TV system is exposed to environmental forces since it consists of metal clad coaxial cables and signal conditioning equipment that is physically spread throughout a geographical service area either aerially on utility poles or buried in the ground. One such environmental force is the discharge of atmospheric electrical charge known as lightening. It is well known that such charge currents can travel along the outer conductor of the coaxial cable, and that periodic connection of the cable to the power utility ground wire and also to ground rods buried in the earth are required to keep the power utility ground and the Cable System ground at the same voltage potential to insure public safety and protect subscriber equipment. However, currents that travel along the outer conductor of a coaxial cable will induce currents on the center conductor of the cable. These electrical disturbances on the center conductor of the coaxial cable, which may be induced by lightening discharge or by switching transients on the power grid, can damage the electrical components connected to the Cable TV system. Therefore, various methods of surge protection have evolved to keep power and communication utilities operating during atmospheric disturbances that may impact either network's distribution system at any point.
A standard developed by the Institute of Electrical and Electronic Engineers, based on empirical studies of typical voltage disturbances, defines standard voltage and current waveforms that any device in the Cable TV system or the electrical power grid may experience. The IEEE C62.41 1999 standard specifies service categories relative to the network architecture and defines the maximum voltage and current expected in these service categories.
Typically, devices connected to the CATV outside plant are classified as category B3 where the voltage may reach 6 kilovolts in an open circuit and the current into a short circuit may reach 3 kiloamps. The waveform is unipolar and is known as a “combination” waveform.
Devices connected inside the customer premises are usually classified as category A3 where the voltage may reach 6 kilovolts, but the waveform is a damped sinusoid known as a “ring” wave. Currents typically are limited by the impedance of the path to ground and typically may reach 500 amperes. These surge waveform standards make it possible to standardize and test the ability of a device in the network to survive the conditions expected for its position in the CATV network.
The Combination Wave is specified in paragraph 9.4.2 of IEEE C62.41. The 1.2/50-8/20 μs (microsecond) combination wave is defined by both an open circuit voltage waveform and a short circuit current waveform. The open circuit voltage waveform has a front time of 1.2 μs and duration of 50 μs (see FIG. 1). The short circuit current waveform has a front time of 8 μs and duration of 20 μs (see FIG. 2).
The Ring Wave is specified in paragraph 9.4.1 of IEEE C62.41. The 0.5 μs-100 kHz (kilohertz) ring wave has an initial rise time of 0.5 μs and an oscillating frequency of 100 kHz, where the frequency is calculated from the first and third zero crossing after the initial peak (see FIG. 3). The current associated with this voltage is determined by the source impedance, which is 12 ohms for a category A3 ring wave, and 2 ohms for a category B3 ring wave.
The methods used to protect devices connected to the CATV system depend upon several factors. First, the impedance of the protected circuit and the frequency at which it functions differentiate the types of surge protection devices that can be used effectively. Internal Power Regulators found in active circuits that require power such as amplifiers usually are protected by devices that function like zener diodes. These devices have a high impedance below a threshold voltage. This enables power to flow to the circuit without being affected by the protection device. Above this threshold voltage, current flows through the protection device to ground thus limiting further voltage rise that could be detrimental to the regulator circuit. Other threshold devices known as crowbar circuits become short circuits themselves when the threshold voltage is exceeded. The gas tube diode and triac semiconductor devices are in this group.
If the protected circuit is a path for high frequency signals, it is not possible to use these types of protection devices. With the exception of the gas tube, they all have a low impedance at radio frequencies that would shunt the communication signals to ground making the device ineffective under any condition. When powering an active device through a port that also carries radio frequency signals, an inductor must be in series with the power regulator input to mask its low impedance from the signal path with a high impedance at radio frequencies.
One common means of protecting a high frequency circuit from low frequency surges is to shunt the path with an inductor to ground that has a high impedance in the operating frequency range of the device but a low impedance below that frequency range. In a CATV system, the lowest frequency is 5 megahertz. A coupling capacitor typically follows the inductor in series with the signal flow into the device. The combination of the capacitor and inductor creates a simple highpass filter. Inductor values in this case are typically 6-8 microhenries and the capacitor value is typically about 1,000 picofarads.
But there are cases, especially in passive splitting devices, where the use of an inductor to protect the circuit is not desirable either because it introduces group delay at the lower band edge, or because it introduces extra cost in a competitive commodity market. In this case, the coupling capacitor takes the full impact of the surge. This places high demands upon the voltage breakdown rating of the dielectric in the capacitor. A high voltage capacitor with the temperature stability required to insure consistent signal parameters at low temperatures is typically larger than the economically sized housing of the device will allow.